1. Field of the Invention
The present invention relates to a drift measuring method, a charged particle beam writing method, and a charged particle beam writing apparatus.
2. Background Art
Recently, along with the development of higher levels of integration in semiconductor integrated circuits, there has been a trend toward finer and more complicated LSI (Large Scale Integration) patterns. Therefore, effort is being made to develop an electron beam lithographic technique for writing a pattern with an electron beam directly instead of using photolithography.
Electron beam lithography inherently provides a superior resolution, since it uses electron beams, which are a type of charged particle beam. This technology is also advantageous in that great depth of focus is obtained, which enables dimensional variations to be reduced even when a large step feature is encountered. For this reason, the technology has been applied to the development of state-of-the-art devices typified by DRAM, as well as to the production of some ASICs. Further, electron beam lithography is widely used in the manufacture of masks or reticles used as original artwork for transferring an LSI pattern to the wafer.
Japanese Laid-Open Patent Publication No. 9-293670 (1997) discloses a variable shape electron beam writing apparatus used for electron beam lithography.
The pattern writing data for such apparatus is prepared by using design data (CAD data) of a semiconductor integrated circuit, etc. designed by a CAD system and processing it, such as correcting the data and dividing the pattern. For example, the pattern is divided into segments each the size of the maximum shot size, which is defined by the size of the electron beam. After this division of the pattern, the apparatus sets the coordinate positions and size of each shot and the radiation time. Pattern writing data is then produced which is used to shape the shot in accordance with the shape and size of the pattern or pattern segment to be written. The pattern writing data is divided into pieces each corresponding to a strip-shaped frame (or main deflection region), and each frame is divided into sub-deflection regions. That is, the pattern writing data for the entire chip has a hierarchical data structure in which data of each of a plurality of strip-shaped frames, which correspond to the main reflection regions, is divided into a plurality of pieces of data each representing one of the plurality of sub-reflection regions (smaller in size than the main deflection regions) in the frame.
The electron beam is scanned over each sub-deflection region by the sub-deflector at higher speed than it is scanned over each main deflection region; the sub-deflection regions are generally the smallest writing fields. When writing on each sub-deflection region, the shaping deflector forms a shot of a size and shape corresponding to the pattern or pattern segment to be written. Specifically, the electron beam emitted from the electron gun is shaped into a rectangular shape by a first aperture and then projected to a second aperture by the shaping deflector, resulting in a change in the shape and size of the beam. The electron beam is then deflected by the sub-deflector and the main deflector and directed onto the mask mounted on the stage, as described above.
Incidentally, irradiating the mask with the electron beam results in generation of reflected electrons. These reflected electrons impinge onto the optical system, detectors, etc. in the electron beam writing apparatus, and as a result, charges are built up, thereby generating a new electric field. This changes the path of the electron beam that has been deflected toward the mask, resulting in displacement of the beam from the desired impinging position or writing position on the mask, which is referred to as “beam drift.” Although other problems can cause beam drift, in any case it is necessary to make corrections to cause the beam to impinge at the desired location on the mask by detecting the reference mark position on the stage in the middle of the write operation and determining the amount of beam drift. Specifically, the coordinates of the reference mark are measured twice; once immediately before initiating a write operation and once after temporarily interrupting the write operation during the process. The difference between these measured coordinate values is then calculated to determine the amount of drift.
A common conventional method for measuring the position of the reference mark is to scan an electron beam over the reference mark and measure the intensity of the reflected electrons. With this method, however, it takes approximately 0.5 milliseconds to obtain each piece of data, including the time required for communication. As a result, the conventional method has not been able to measure beam drift in a time on the order of microseconds.
The present invention has been made in view of the above problems. It is, therefore, an object of the present invention to provide a drift measuring method, a charged particle beam writing method, and a charged particle beam writing apparatus capable of measuring beam drift in a shorter time than conventionally possible.
Other challenges and advantages of the present invention are apparent from the following description.